Processes for purifying phosphoric acid and for preparing phosphoric acid

ABSTRACT

A process for purifying phosphoric acid includes removing volatile components from phosphoric acid rich in impurities to form a crude phosphoric acid liquid substantially free of volatile components; heating the crude phosphoric acid liquid to a temperature above 250° C. in order to decompose phosphoric acid in the crude phosphoric acid liquid and generate gaseous phosphoric anhydride; and introducing the gaseous phosphoric anhydride into water or a dilute phosphoric acid aqueous solution in order to hydrate the gaseous phosphoric anhydride to form phosphoric acid.

This application is a continuation application of U.S. application Ser.No. 10/916,395, filed Aug. 12, 2004 now U.S. Pat. No. 7,083,768 (ofwhich the entire disclosure of the pending, prior application is herebyincorporated by reference).

FIELD OF THE INVENTION

The present invention relates to processes for purifying phosphoric acidand for producing phosphoric acid, and particularly to processes forpurifying phosphoric acid and for producing phosphoric acid by hydratinggaseous phosphoric anhydride.

BACKGROUND OF THE INVENTION

Traditionally, the methods for purifying phosphoric acid include anextraction method, e.g. the method disclosed in U.S. Pat. No. 6,251,351extracting phosphoric acid produced by the wet process withN,N-dialkyl-α-aminoacid in order to increase the purity of phosphoricacid; a precipitation method, e.g. the method diclosed in U.S. Pat. No.4,986,970 using dithio carbonic acid-O-ester to separate heavy metals byprecipitation; and an ion-exchange method, e.g. the method disclosed inU.S. Pat. No. 5,006,319 using a strong alkaline anionic exchange resinto remove metal ions from phosphoric acid. However, the quality ofphosphoric acid produced by these methods is far inferior than thequality of phosphoric acid produced by the dry process. Thus, thesemethods are rarely used in the industries. U.S. Pat. No. 4,495,165discloses a purification method for phosphoric acid produced by the wetprocess including absorbing the phosphoric acid produced by the wetprocess with porous sawdust, heating the phosphoric acid absorbed in thesawdust to form polyphosphoric acid and/or phosphoric acid esters, andusing water to hydrate the polyphosphoric acid and/or phosphoric estersin the porous sawdust into phosphoric acid. According to this process,about 90% to 95% of the impurities in the phosphoric acid produced bythe wet process remain in the porous sawdust. Thus, a partialpurification effect can be achieved. However, said purification methodis a batch process and has an extremely high cost. Furthermore, thepurity of phosphoric acid produced by said method is still far inferiorthan that of the dry process. For example, the iron content in thephosphoric acid produced by the dry process is less than 10 ppm, and theiron content in phosphoric acid purified by said method is higher than100 ppm.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a process forpurifying phosphoric acid.

Another objective of the present invention is to provide a process forpurifying phosphoric acid by decomposing phosphoric acid containingimpurities into gaseous phosphoric anhydride, and hydrating saidphosphoric anhydride into phosphoric acid.

Another objective of the present invention is to provide a process forpurifying phosphoric acid by using the combustion heat of phosphor todecompose phosphoric acid containing impurities into gaseous phosphoricanhydride, and hydrating said phosphoric anhydride into phosphoric acid.

Still another objective of the present invention is to provide acomprehensive phosphoric acid production process by combining a dryprocess for producing phosphoric acid and a waste phosphoric acidpurification process.

A further objective of the present invention is to provide acomprehensive phosphoric acid production process by combining a dryprocess for producing phosphoric acid and a purification process Forphosphoric acid produced by a wet process.

The inventors of the present invention heat the impurities-richphosphoric acid after the removed of volatile components containedtherein to decompose phosphoric acid and generate gaseous phosphoricanhydride; and introduce said gaseous phosphoric anhydride into water ora dilute phosphoric acid aqueous solution to hydrate said gaseousphosphoric anhydride to form phosphoric acid. The first step of thepresent invention removes the volatile components from theimpurities-rich phosphoric acid so that a crude phosphoric acid liquidsubstantially free of volatile organic or inorganic impurities isobtained. The step of generating gaseous phosphoric anhydride is aimedto let non-volatile organic or inorganic impurities remain in a viscousresidue solution, so that a high purity phosphoric anhydridesubstantially free of volatile and non-volatile organic or inorganicimpurities can be hydrated to phosphoric acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a graphite heating apparatus accordingto the present invention;

FIG. 2 is a schematic diagram of a rotary heating evaporation furnacedevice according to the present invention;

FIG. 3 is a schematic diagram of a phosphorus combustion chamberaccording to the present invention.

Description of the numerals of the key elements 10 Graphite heatingchamber 20 Cover 30 Ventilation tube 40 Electric furnace 50 Rotaryfurnace body 60, 90, 130, Conduits 140, 160 70 Combustion device 80Ventilation tube 110 Phosphorus combustion chamber furnace body 120Cover of furnace body 150 Nozzle

DETAILED DESCRIPTION OF THE INVENTION

A method for purifying phosphoric acid according to the presentinvention comprises:

removing volatile components from phosphoric acid rich in impurities toform a crude phosphoric acid liquid substantially free of volatilecomponents;

heating said crude phosphoric acid liquid to a temperature above 250° C.in order to decompose phosphoric acid in said crude phosphoric acidliquid and generate gaseous phosphoric anhydride; and

introducing said gaseous phosphoric anhydride into water or a dilutephosphoric acid aqueous solution in order to hydrate said gaseousphosphoric anhydride to form phosphoric acid.

Said phosphoric acid rich in impurities can be a phosphoric acid productfrom a wet process, waste phosphoric acid, or waste miscellaneous acidcontaining phosphoric acid, wherein the components and amounts of theimpurities vary depending on the source thereof and may include volatilecomponents, organic impurities, and metal ions, etc.

Said volatile components can be arbitrary volatile components, e.g.nitric acid, acetic acid, etc. A certain amount of water may also beremoved when removing the volatile components.

Said step of removing the volatile components can be any conventionalprocess in removing volatile components, so that substantially all thevolatile components in the phosphoric acid rich in impurities areevaporated into gas, while a certain amount of water is still retainedso that an un-evaporated portion is still in a liquid form (the crudephosphoric acid liquid). For example, a heating process can be used toevaporate the volatile components in the phosphoric acid rich inimpurities into gas to escape therefrom, and preferably said step ofremoving volatile components comprises heating to a temperature above80° C., more preferably above 100° C. For example, an aerated heatingprocess can be used to heat the phosphoric acid rich in impurities whilean insoluble gas (e.g. air) is being introduced to entrain the volatilecomponents from the phosphoric acid rich in impurities. For example, avacuum heating process can be used to accelerate the evacuation ofvolatile components from the phosphoric acid rich in impurities byheating the phosphoric acid rich in impurities in vacuo (the vacuumlevel being determined according to the actual requirement). Generallyspeaking, the vacuum heating process and the aerated heating process arepreferable, and the aerated heating process is particularly preferable.When necessary, a multi-stage process can be used to remove the volatilecomponents. For example, nitric acid and a portion of water are removedat about 80° C., and acetic acid and a portion of water are removed atabout 100° C.

Said step of heating said crude phosphoric acid liquid to generategaseous phosphoric anhydride by decomposition can be any conventionalheating process, e.g. an electric heating, a combustion heatingincluding an indirect combustion heating, and a direct combustionheating, wherein the direct combustion heating process is preferable. Aphosphorus combustion direct heating process is more preferable, whichincludes introducing said crude phosphoric acid liquid into a phosphoruscombustion chamber where phosphorus is combusted so that a combustionheat of phosphorus is used to heat and decompose said crude phosphoricacid liquid into gaseous phosphoric anhydride. Preferably, said crudephosphoric acid liquid is introduced into said combustion chamber in amanner so that a film of said crude phosphoric acid liquid is formed ona wall of the combustion chamber, and is heated and decomposed togenerate gaseous phosphoric anhydride.

Said phosphorus combustion direct heating process is a dry process forproducing phosphoric acid, where phosphorus is combusted in thecombustion chamber to form gaseous phosphoric anhydride. Said combustionof phosphorus to form gaseous anhydride includes introducing phosphorusand an oxygen-containing gas into the combustion chamber, wherein thecombustion conditions and combustion devices thereof are all similar tothose of the conventional dry process for producing phosphoric acid.However, since the introduction of the crude phosphoric acid liquidaccording to the process of the present invention will consume a portionor even a major portion of the phosphorus combustion heat, thetemperature of the combustion chamber will decrease dramatically.Generally speaking, the temperature in the combustion chamber of theconventional dry process for producing phosphoric acid is 1200° C. to1800° C. However, the average temperature of the combustion chamberaccording to the present invention is 250° C. to 1000° C., preferably300° C. to 800° C., more preferably 350° C. to 600° C. Since thetemperature of the combustion chamber can be dramatically reduced, thedesign of a combustion chamber according to the present invention iseasier. In particular, the advantages of the present invention includelower criteria on the selection of material for the combustion chamber,without the need of a heat dissipation design, and a lower equipmentcost, etc. That is an average temperature of said gaseous phosphoricanhydride is kept at 300 to 800° C., preferably 350° C. to 600° C.,during said step of heating said crude phosphoric acid liquid togenerate gaseous phosphoric anhydride by decomposition.

Said step of introducing the gaseous anhydride into water or a dilutephosphoric acid aqueous solution to hydrate said gaseous anhydride intophosphoric acid can be carried out by the method and device adopted bythe conventional dry process for producing phosphoric acid. Preferably,said step of introducing the gaseous anhydride into water or a dilutephosphoric acid aqueous solution to hydrate said gaseous anhydride intophosphoric acid comprises introducing said gaseous phosphoric anhydridegenerated by decomposition and the gaseous phosphoric anhydrideresulting from the combustion of phosphorus into water or said dilutephosphoric acid aqueous solution to hydrate said gaseous phosphoricanhydride and form phosphoric acid. Generally speaking, the use of adilute phosphoric acid aqueous solution to hydrate said gaseousanhydride is preferable.

The present invention also discloses a process for producing phosphoricacid, which comprises the purification process of the present invention.

The present invention will be better understood through the followingexamples which are illustrative only and not for limiting the scope ofthe present invention.

EXAMPLE 1

1000 g of a waste acid from the photoelectric industry was used, and itsmain components are shown in Table 1.

TABLE 1 Main components of a waste acid from the photoelectric industryPhosphoric acid 68.7%  Acetic acid 7.2% Nitric acid 3.4% Aluminum0.048% 

Poured the waste acid in a pyriform flask of a rotary evaporator. Thepyriform flask was slantly mounted in a water bath at 90° C., androtated at about 60 rpm while being vacuumed to remove the acetic acidand nitric acid from the waste acid. After 60 minutes, 745 g of a crudephosphoric acid liquid was obtained, the composition of which wasanalyzed and is shown in Table 2.

TABLE 2 Main ingredients of the crude phosphoric acid liquid Phosphoricacid   92% Acetic acid N/D* Nitric acid N/D Aluminum 0.064%*N/D—non-detected, hereinafter abbreviated as N/D

500 g of the crude phosphoric acid liquid was placed in the apparatusshown in FIG. 1, wherein 10 represents a graphite heating chamber, 20represents a cover, and 30 represents a ventilation tube. Said graphiteheating chamber 10 was heated in an electric furnace 40 in order todecompose the crude phosphoric acid liquid into phosphoric anhydride.The phosphoric anhydride was introduced into 100 ml of water through theventilation tube 30 on the cover 20, thereby obtaining 530 g ofphosphoric acid having a composition shown in Table 3.

TABLE 3 Main composition of the phosphoric acid Phosphoric acid 81.7%Acetic acid N/D Nitric acid N/D Aluminum 1.2 ppm

EXAMPLE 2

As shown in FIG. 2, 50 represents a rotary furnace. A waste phosphoricacid was introduced into the furnace through a conduit 60. 70 representsa combustion device using an ordinary fuel. The combustion flame fromthe combustion device 70 was introduced into the furnace so that thewaste phosphoric acid solution in the form of a film on the wall of therotary furnace was heated directly. The volatile gas resulting fromevaporation was evacuated through a ventilation tube 80. The crudephosphoric acid depleted of the volatile components flew out through aconduit 90. In the present example, the fuel to the combustion devicewas controlled to adjust the temperature of the crude phosphoric acidflowing out from the conduit 90. The analysis results in this exampleare shown in Table 4.

TABLE 4 Analysis results of crude phosphoric acid Samples at varioustemp. Waste 80° C. 100° C. 120° C. 150° C. phosphoric acid Crude CrudeCrude Crude before entering phosphoric phosphoric phosphoric phosphoricItems of analysis the furnace acid acid acid acid Phosphoric acid 68.782.3 86.5 89.8 96.8 (%) Acetic acid (%) 7.2 4.5 0.3 — — Nitric acid (%)3.4 0.2 — — — Aluminum 480 570 615 631 686 (ppm)

EXAMPLES 3–7

As shown in FIG. 3, 110 represents the furnace body of a phosphoruscombustion chamber and 120 represents an upper cover of the furnacebody. A crude phosphoric acid at 150° C. from Example 2 was directlyintroduced through a conduit 130 into the upper end of the furnace body,from which it overflew to form a uniform liquid film of the crudephosphoric acid on the furnace wall. Phosphorus was sprayed into thefurnace for combustion through a nozzle 150, and the required air wasintroduced through a conduit 160. The liquid film of the crudephosphoric acid liquid was directly heated by the combustion heat fromthe combustion of phosphorus, and was decomposed into gaseous phosphoricanhydride, which was introduced through a conduit 140 to a hydrationabsorption process in the dry process for producing phosphoric acid tobe hydrated with pure water into clean phosphoric acid. The residualcrude phosphoric acid liquid was discharged from a conduit 170 at thebottom of the furnace. The results of these examples are shown in Table5.

TABLE 5 Results of Examples 3–7 Amount of Amount of Amount of crudehydrated Conc. of aluminum in phosphoric Amount of Temp. of phosphorichydrated the hydrated acid phosphorus gaseous acid phosphoric phosphoricintroduced introduced phosphoric produced acid acid Example (kg) (kg)anhydride (kg) (%) (ppm) 3 1732 142 365° C. 1945 83.0 1.32 4 1590 145421° C. 1890 85.2 0.84 5 1390 146 502° C. 1825 86.0 0.52 6 1246 148 571°C. 1736 86.3 0.96 7 1143 150 626° C. 1650 86.5 1.06

EXAMPLES 8–12

The procedures in Examples 3–7 were repeated, except that natural gaswas used as fuel to replace phosphorus. The results of these examplesare shown in Table 6.

TABLE 6 Results of Examples 8–12 Amount of Amount of Amount of crudehydrated Conc. of aluminum in phosphoric Amount of Temp. of phosphorichydrated the hydrated acid natural gas gaseous acid phosphoricphosphoric introduced introduced phosphoric produced acid acid Example(kg) (kg) anhydride (kg) (%) (ppm) 8 1802 89 381° C. 1617 85.2 1.35 91795 102 468° C. 1660 86.1 1.36 10 1790 115 512° C. 1742 86.0 2.45 111800 128 590° C. 1769 86.2 3.1 12 1785 141 645° C. 1763 86.3 3.2

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except as and to the extent that they are included in theaccompanying claims. Many modifications and variations are possible inlight of the above disclosure.

1. A process for producing phosphoric acid comprising a purificationprocess, wherein said purifying process comprises the following steps:removing volatile components from phosphoric acid rich in impurities,wherein said phosphoric acid rich in impurities is a phosphoric acidproduct produced by a wet process, to form a crude phosphoric acidliquid substantially free of volatile components; heating said crudephosphoric acid liquid to a temperature above 250° C. in order todecompose phosphoric acid in said crude phosphoric acid liquid andgenerate gaseous phosphoric anhydride; and introducing said gaseousphosphoric anhydride into water or a dilute phosphoric acid aqueoussolution in order to hydrate said gaseous phosphoric anhydride to formphosphoric acid, wherein said step of heating said crude phosphoric acidliquid comprises heating said crude phosphoric acid liquid with acombustion heat comprising introducing said crude phosphoric acid liquidinto a phosphorus combustion chamber where phosphorus is combusted. 2.The process as claimed in claim 1, wherein said step of removingvolatile components comprises heating to a temperature above 80° C. 3.The process as claimed in claim 2, wherein said step of removingvolatile components comprises heating to a temperature above 100° C. 4.The process as claimed in claim 3, wherein said step of removingvolatile components comprises heating the phosphoric acid rich inimpurities in vacuo or heating the phosphoric acid rich in impuritieswhile air is being introduced to entrain the volatile components fromthe phosphoric acid rich in impurities.
 5. The process as claimed inclaim 1, wherein said step of introducing the gaseous anhydride intowater or a dilute phosphoric acid aqueous solution to hydrate saidgaseous anhydride into phosphoric acid comprises introducing saidgaseous phosphoric anhydride generated by decomposition and a gaseousphosphoric anhydride resulting from the combustion of phosphorus intowater or said dilute phosphoric acid aqueous solution to hydrate saidgaseous phosphoric anhydride and form phosphoric acid.
 6. The process asclaimed in claim 1, wherein said crude phosphoric acid liquid isintroduced into said phosphorus combustion chamber in a manner so that afilm of said crude phosphoric acid liquid is formed on a wall of thecombustion chamber, and is heated and decomposed to generate gaseousphosphoric anhydride.
 7. The process as claimed in claim 6, wherein anaverage temperature of said gaseous phosphoric anhydride is kept at 300to 800° C. during said step of heating said crude phosphoric acid liquidto generate gaseous phosphoric anhydride by decomposition.
 8. Theprocess as claimed in claim 7, wherein the average temperature of saidgaseous phosphoric anhydride is kept at 350 to 600° C.
 9. The process asclaimed in claim 8, wherein said hydration step comprises using saiddilute phosphoric acid aqueous solution to hydrate said gaseousphosphoric anhydride into phosphoric acid.
 10. The process as claimed inclaim 5, wherein an average temperature of said gaseous phosphoricanhydride is kept at 300 to 800° C. during said step of heating saidcrude phosphoric acid liquid to generate gaseous phosphoric anhydride bydecomposition.
 11. The process as claimed in claim 10, wherein theaverage temperature of said gaseous phosphoric anhydride is kept at 350to 600° C.
 12. The process as claimed in claim 11, wherein saidhydration step comprises using said dilute phosphoric acid aqueoussolution to hydrate said gaseous phosphoric anhydride into phosphoricacid.